Viral Structure and Morphology

Viral Structure and Morphology

Viruses are obligate intracellular parasites. This means they can only replicate inside a host cell, as they lack the ability to produce their own building blocks or synthesize proteins.

Viral Structure:

• Capsid: This is the protein coat surrounding the virus, composed of protein subunits called capsomeres. Capsomeres are often symmetrical and identical.

• Envelope: This is a lipid membrane surrounding the virus, which may contain glycoproteins.

• Virion: This refers to the complete viral structure, including the nucleocapsid (which consists of the genome and capsid) or the nucleocapsid and envelope.

Viral Genome:

• Viruses use complex methods to invade host cells, take control of their cellular machinery, and replicate themselves.

• The viral genome can be either DNA or RNA, single-stranded or double-stranded, linear or circular.

• Examples:

• Herpes and smallpox viruses have double-stranded DNA genomes.

• Influenza viruses have segmented genomes consisting of eight single-stranded RNA segments.

• Poliovirus has a single-stranded RNA genome.

Viral Replication:

• Viruses replicate through assembly, using host cell enzymes and organelles.

Viral Classification:

• Viruses are classified based on their ability to cause disease, their structure, virion morphology, genome structure, and replication methods.

• Viral families are named ending in “viridae,” for example: Herpesviridae, Orthomyxoviridae.

• Viral genera are named ending in “virus,” for example: Herpes simplex virus, Influenza A virus.

Viral Morphology:

• Viruses exhibit a variety of shapes:

• Icosahedral: Adenovirus

• Helical: Influenza virus

• Complex: Smallpox virus

Viral Size:

• Viruses typically range in size from 20-300 nm.

Functions of the Capsid:

• Maintains viral shape and size.

• Protects the nucleic acid.

• Helps non-enveloped viruses attach to host cells.

• Acts as a specific viral antigen.

Envelope:

• The envelope can be composed of phospholipids and glycoproteins.

• Glycoproteins may contain spacer proteins that maintain a distance between the envelope and the capsid.

• The envelope serves as part of the receptor binding system.

• It does not play a role in phagocytosis or active transport.

Functions of Envelope Proteins or Glycoproteins:

• Attachment to host cells.

• Specific antigens (which can mutate as in influenza viruses).

• Morphology.

• Assembly and release of viruses.

Enveloped Viruses are Susceptible to:

• Ether and other organic solvents.

• Loss of infectivity when the envelope is removed.

• They are not stable in the gastrointestinal environment and are typically transmitted through secretions, blood, and organ transplants.

Non-enveloped Viruses:

• They are stable in the host environment and unaffected by heat, acid, etc., making them easily transmissible via the digestive tract.

Defective Viruses:

• They lack one or more functional genes required for viral growth.

• They require assistance from another virus at some stage of their life cycle.

Defective Interfering Viruses:

• These are deletion mutants that interfere with the replication of wild-type viruses.

• They play a role in the formation and maintenance of persistent viral infections.

Virus-Assisted Viruses:

• Hepatitis D virus can only replicate in the presence of Hepatitis B virus.

• This is because the D virus’s nucleic acid is enveloped by the B virus’s membrane.

Pseudovirion:

• It contains more host DNA than viral genome.

• It lacks the ability to replicate and produce new viruses.

• It can transfer nucleic acid from one cell to another.

Examples of Pseudovirion:

• Retroviruses.

• They have the potential to generate oncogenes.

Viroids:

• They occur in plants.

• They are single-stranded circular RNA but appear linear due to intramolecular hydrogen bonding.

• They do not encode for any protein products.

Prions:

• They are not viruses because they lack a nucleic acid component.

• They are abnormally folded proteins that cause disease.

Prions are associated with a number of diseases:

• Bovine spongiform encephalopathy (mad cow disease).

• Scrapie in sheep.

• Chronic wasting disease in deer.

• Creutzfeldt-Jakob disease in humans.

Methods to Destroy Prions:

• Cooking cannot inactivate prions.

• They are destroyed by high temperatures or autoclaving using concentrated NaOH.

Prion Treatment:

• There is no specific treatment for prion diseases.

• Anti-malaria drugs and psychotropic drugs have been shown to destroy prions in mice.

Prions are thought to contribute to some neurological diseases such as:

• Parkinson’s disease, Huntington’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis.

• They also play a role in type II diabetes and cancer.